1. Field of the Invention
This invention relates to a bridge apparatus in which, in a local area network (LAN), a plurality of branch lines LANs are connected by a trunk line LAN.
2. Description of the Prior Art
Generally, in the case where similar branch line LANs for performing packet switching are connected to a trunk line LAN for performing packet switching, thereby carrying out communication, it is necessary to select frames on the trunk line LAN and input the frames into the branch lines, and select frames on the branch lines and transmit the frames on the trunk line. For this, a mechanism such as an address filter for determining the necessity of relaying dependent on a destination address of the frame is attached to a bridge apparatus. This address filter has a data base called an address table, and judges the necessity of relaying the frame depending on whether the destination address of the frame is found on the address table.
FIG. 7 is a block diagram showing a conventional bridge apparatus. In FIG. 7, reference numeral 3 is a communication control apparatus of a trunk line LAN, 4 a communication control apparatus of a branch line LAN, 5 an address table which stores addresses of terminals existing on the branch line as a branch address table 5A, 6 a control apparatus for the communication apparatus, which includes a microcomputer 6A and a memory 6B.
Also, FIG. 8 is a constitutional concept diagram of a network which is constituted using the bridge apparatus. In FIG. 8, reference numeral 1 is a trunk line LAN, reference numerals 21 to 23 are branch line LANs, a to f are terminals connected to the respective branch line LANs 21 to 23, 31 to 33 are communication control apparatus in the trunk line LAN 1, 41 to 43 are communication control apparatus of the branch line LANs 21 to 23, 5A1 to 5A3 are address tables for the respective branch line LANs 21 to 23, and 61 to 63 are control apparatus for these apparatus.
FIG. 9 shows an example of a frame of data used in the LAN. Reference numeral 7C is an address of a destination terminal, and the destination address (DESTINATION ADDRESS) is usually abbreviated and called DA. Reference numeral 7D is an address of a source terminal, and the source address (SOURCE ADDRESS) is usually abbreviated and called SA. Reference numeral 7E are data which a frame carries.
Next, the operation of the bridge apparatus will be described with reference to the flowcharts of FIG. 10 and FIG. 11.
At first, in FIG. 10, the frame receiving operation from the trunk line LAN 1 to the control apparatus (61 to 63 in FIG. 8) will be described.
At first, it is judged whether the destination address of the frame belongs to the terminals a to f which exist in the branch line network. (Step ST1). When the destination address agrees with the branch line address 5A (In FIG. 8, any one of the branch address tables 5A1 to 5A3), the frame is relayed to the branch line side by a relay operation (Step ST2), and the frame is received by a predetermined one of the terminals a to f on the branch lines LAN 21 to 23, respectively.
FIG. 11 represents the receiving operation of frames from the branch line LANs 21 to 23 to the control apparatus. According to the receiving operation, when the operation is begun by reception of the frames, it is checked whether the source address SA7D of the frame,agrees with the address table 5A (Step ST3). If the addresses do not agree with each other, the source address is added to the address table 5A anew. (Step ST4). On the other hand, when both agree with each other, it is checked whether the destination address DA7C of the frame exists in the address table 5A (Step ST5). If the destination address exists, it indicates that the frame is one between terminals in the same branch line, and the frame is "scrapped" or discarded from the memory of the control apparatus (Step ST6). If the destination address does not exist, it is judged that the frame is for a terminal on an external branch line, and the frame is relayed to the trunk line LAN 1 (Step ST7).
Since the conventional bridge apparatus is constituted as described above, the database of address table 5A is formed by source addresses of frames received from the branch line LANs 21 to 23. Accordingly, since the address of a terminal which has not originated transmission of a frame is not registered in the address table 5A, a frame from the trunk line intended for such terminal LAN 1 can not be received. In other words, in FIG. 8, though addresses of the terminals a, c, and e are registered, even if, for instance, the terminal e sends a frame to the terminal b, the frame is not relayed to the branch line LAN 21 because the address of the terminal b is not registered in the address table 5A. Accordingly, the terminal b can not receive the frame.
In this way, the conventional bridge apparatus does originate some calls and has the problem that the terminals a to f of which addresses have not been registered can not receive frames from other branch lines LAN 21 to 23.